A spiral wound module is the most common configuration for reverse osmosis and nanofiltration membranes. In operation, “Feed” liquid under pressure enters the module at one end, flows axially through a feed spacer sheet, and exits on the opposite end as “concentrate”. “Permeate” solution (commonly water) passes under pressure through the membrane while a solute (often salt) is substantially retained. The spiral wound configuration allows a large amount of membrane area to be packed into a small volume.
One or more membrane envelopes and feed spacer sheets are wrapped about a central permeate collection tube. The envelopes comprise two generally rectangular membrane sheets surrounding a permeate carrier sheet. This “sandwich” structure is held together by an adhesive along three edges of each membrane sheet: the back edge furthest from the permeate tube, and the two side edges that will become the feed (inlet) and concentrate (outlet) ends of the module. Adhesive at the side edges additionally affix and seal membrane sheets to the permeate tube at each end of the module. The fourth edge of the envelope is open and abuts the permeate collection tube so that the permeate carrier sheet is in fluid contact with small holes passing through the permeate collection tube. Construction of spiral wound modules is described further in U.S. Pat. Nos. 5,538,642, 5,681,467, and 6,632,356, which are incorporated by reference.
The time and complexity associated with fabricating a module increases with the number of membrane envelopes used in its construction. Since all envelopes in a module are wound together in the last step of rolling, it is important that adhesive applied to a first leaf is not cured before the last leaf is inserted. Whether rolling manually or using automation, it is further desirable that the time for solidifying adhesive lines is substantially longer than the time minimally required for constructing the module, since this allows for potential upsets or delays in the process that would otherwise scrap a module. As described in U.S. Pat. No. 5,096,584, particularly suitable adhesives for joining membrane leaves are “. . . commercially-available polymeric adhesives, e.g. H.P. Fuller polyurethane or Dow epoxy material (DER) which is cured with a diamine, and sets up as a flexible solid with a variable cure time, typically about 2-24 hours or so.”
After adhesive cures, the two opposing ends of the module are optionally trimmed to remove any excess adhesive that might extend beyond the sides of the membrane sheets. The trimming procedure may be performed while rotating the module. Cuts at each end are made from the outer diameter of the module to approximately the outer diameter of the permeate tube. The lines of adhesive running along both side edges of each membrane sheet are typically cross sectioned in the trimming process, but the cuts must not extend into the permeate region. Trimming results in a well-defined module length and a smooth scroll face.
In a defect free module, the membrane barrier layer effectively separates feed solution from permeate liquid. However, there are several regions of a module where feed solution may potentially leak into the permeate flow path. The membrane itself may have localized defects such as scratches and pinholes. Broken or missing adhesive lines running along the back and sides of the permeate carrier sheet can provide a direct path for feed to enter the permeate. At the inlet and outlet ends of the module, within a few millimeters of the permeate tube, voids may exist where adhesive surrounding the tube fails to effectively seal the feed solution from the permeate flow path. Finally, the folded edge abutting the permeate collection tube has been a common source of leaks, particularly for modules subjected to very rigorous and frequent cleaning cycles.
The trimming process has potential to create large leaks in a module that would otherwise be sealed, particularly in the area of adhesive that affixes membrane sheets to the permeate tube. Even when the trimming process appropriately avoids slicing into the permeate region, the act of cross sectioning the adhesive can open otherwise sealed voids and allow for fluid communication between feed and permeate solutions. The original defect may result from a leaf that was improperly inserted or, especially, from a leaf that pulls away from the module during construction and creates a gap within the adhesive. In either case, a gap near the permeate tube that allows feed fluid to pass into the permeate channel is referred to here as an “insertion-point leak.”
A typical spiral wound module for seawater desalination will pass less than 0.3% NaCl when tested at standard conditions (800 psi, 32000 ppm NaCl feed). A single insertion-point leak can ruin a reverse osmosis module. A one inch long hole of only 0.5 mm in diameter can pass enough feed solution into the permeate region to cause more than 5% salt passage during a standard test.
As illustrated in FIG. 1, some high rejection modules at FilmTec (SW-380-HR) have been further modified by application of a protective bead of sealant to the trimmed ends of a module at the intersection of the permeate tube and scroll face, so that the bead surrounds the permeate tube and reduces the potential for a leak in this area. A sealing bead consisting of a two-part urethane, of the same type used for adhering the three edges of membrane envelopes in the module, has been applied at the intersection of the permeate tube and scroll face for this purpose. Alternatively, a bead of hot melt has been applied in this manner.
In laboratory experiments with simulated leaks, we have found that sealants capable of forming covalent bounds after application were more effective than the hot melt in providing a robust seal. However, use of such reactive adhesives for this purpose introduces several practical issues. Application and curing of an adhesive bead applied about the permeate tube is an additional step after forming a module that further increases production time, so that long cure times, as are typically needed and used in module construction, are undesirable in this process. Long cure times are also undesirable because low viscosity materials, that can best enter and seal small voids, have potential to run under gravity over time. At the other extreme, application of small amounts of reactive materials with very short cure times can be very difficult to implement in an automated, high volume, environment, as small upsets to the process provide the likelihood that polymerization within the applicator could cause substantial downtime.
A sealant material is desired that may be applied as a liquid to the scroll ends of spiral wound modules, in the vicinity of the permeate tube, and that may be rapidly cured by reaction to form covalent bonds and prevent insertion-point leaks. It is further desired that the rate of reaction for the sealant is made to increase after it is applied to the spiral wound module, so that its rate of reaction while in contact with the module is at least twice that of its rate of reaction prior to contact with the module. It is desire that this reactive sealant achieves a stable form in less than 15 minutes. Most preferably, the reactive sealant is made stable in less than 5 minutes or even less than 1 minute.